The present invention relates to an improvement in a pellicle for photolithography or, more particularly, to an improvement in a photolithographic pellicle supported on a frame having an end surface coated with a pressure-sensitive adhesive and protected by applying a releasable protective sheet thereto.
Along with the rapid progress in recent years toward a higher and higher density of integration in semiconductor devices such as ICs, LSIs and the like, the manufacturing process of these semiconductor devices involves a photolithographic patterning process requiring extreme fineness increasing year by year. The photolithographic patterning work involves a step in which a photoresist layer formed on the surface of a substrate is exposed pattern-wise to actinic rays such as ultraviolet light through a pattern-bearing transparency called a photomask. In view of the extreme fineness of the pattern to be reproduced in the photoresist layer, it is essential that the photomask is absolutely free from deposition of any foreign materials such as dust particles because deposition of dust particle on the photomask directly affects the quality of the pattern reproduction.
In this regard, it is a conventional practice that a photomask under the process of photolithographic light-exposure is protected by mounting thereon a frame-supported pellicle which is an integral device consisting of a frame, referred to as a pellicle frame hereinafter, made from a rigid material such as an aluminum alloy and a highly transparent thin film of a plastic resin, referred to as a pellicle membrane hereinafter, spread over and adhesively bonded to one end surface of the pellicle frame in a slack-free fashion. It is usual that the other end surface of the pellicle frame, i.e. opposite to the pellicle membrane is coated with a pressure-sensitive adhesive or a sticking agent in order to facilitate securing of the frame-supported pellicle mounted on the photomask. Further, it is usual, prior to mounting of the frame-supported pellicle on the photomask, that the adhesive-coated end surface of the pellicle frame is protected, in order to prevent deposition of dust particles thereon or stain of other parts with the adhesive, by applying and attaching a releasable sheet of, usually, a plastic resin, referred to as a protective sheet hereinafter, which is removed by peeling immediately before mounting of the frame-supported pellicle on the photomask, to cover the whole adhesive-coated end surface of the pellicle frame.
Needless to say, it is a routine procedure in the manufacturer of frame-supported pellicles that the product finished by attaching a protective sheet to the adhesive-coated end surface of the pellicle frame is, prior to packaging, inspected piece by piece with utmost care to ensure absolute absence of any dust particles deposited on the frame-supported pellicle. Nevertheless, it is sometimes the case that deposition of particles of dust or other foreign material is detected on the pellicle membrane of a frame-supported pellicle taken out of the unpacked package at the user's site. These dust particles naturally can never be other than those produced from somewhere by the mechanical shocks and vibrations during transportation of the packaged products on trucks, railroad cars or airplanes and handling of the packaged products before and after transportation.
The above described problem relative to deposition of dust particles on the pellicle membrane under packaging is old and various proposals have been made heretofore to solve this problem. For example, with an assumption that dormant dust particles are present on surfaces other than the pellicle membrane and eventually move on to the pellicle membrane during, e.g., transportation, Japanese Patent Kokai 60-57841 and 64-48052 propose to form a layer of a sticky adhesive as a dust trap on the side surfaces or, in particular, on the inwardly facing side surface of the pellicle frame or coating the surface with a coating composition standing on a presumption that the side surface of the pellicle frame is the first possibility as a holder of dormant dust particles. When a double-sided pressure-sensitive adhesive tape or film having a foamed structure is used in place of the adhesive layer on one end surface of the pellicle frame, the peripheral section of the tape or film formed by cutting or die-punching is a second possibility as a source for the occurrence of dust particles so that Japanese Patent Kokai-2-64540 proposes to provide a layer of a pressure-sensitive adhesive on such a cut section of the double-sided pressure-sensitive adhesive tape or film. These prior art methods, however, are not always quite effective to prevent occurrence of dust particles on the pellicIe membrane even under a hermetically packaged condition.
Therefore, it is eagerly desired to develop a means to prevent occurrence of dust particles inside of a package of a frame-supported pellicle with a releasable protective sheet attached to the adhesive-coated end surface of the pellicle frame.